1. Field of the Invention
The present invention relates to a stiffness detector that detects stiffness of a paper sheet, a stiffness detection method, and a paper sheet processor including the stiffness detector.
2. Description of the Related Art
A paper sheet processor that counts and discriminates various kinds of paper sheets has been put to practical use. The paper sheet processor takes in paper sheets put in an injection module one by one and carries them to an examination device for paper sheets. The examination device executes various kinds of processing with respect to paper sheets to judge states of the paper sheets. If the paper sheet is, e.g., a paper currency, the paper sheet processor executes a judgment on a type of each paper sheet, a true-false judgment, a judgment upon whether a paper sheet can be again circulated (a wear judgment), and others based on an examination result obtained by the examination device.
The paper sheet processor determines paper sheets having degraded stiffness as paper sheets that are not suitable for recirculation. Therefore, the examination device detects mechanical characteristics such as a degree of deterioration in stiffness of paper sheets.
For example, Jpn. Pat. Appln. KOKAI Publication No. 5-80605 as a Japanese patent document discloses a technology for transmitting acoustic waves to a paper sheet and measuring a weight per unit area of the paper sheet based on a level of reflected waves or transmitted waves.
Further, Jpn. Pat. Appln. KOKAI Publication No. 2008-164394 as a Japanese patent document discloses a technology for causing ultrasonic waves to enter a sample such as a metal plate, receiving leaky waves of waves propagated through the sample, and detecting a defect in the sample based on an amplitude of a received waveform.
In the apparatus having the above-described configuration, for example, when a transmitter that transmits acoustic waves and a receiver that receives acoustic waves are arranged to face each other, crosstalk that acoustic waves output from the transmitter directly enter the receiver without the intermediary of a paper sheet occurs.
The technology disclosed in Jpn. Pat. Appln. KOKAI Publication No. 5-80605 prevents the acoustic waves from entering the receiver without the intermediary of the paper sheet by providing an acoustic absorbent. However, there is a problem that the acoustic waves may possibly enter the receiver when they are diffracted.
Further, as described in Jpn. Pat. Appln. KOKAI Publication No. 5-80605, when receiving transmitted waves or reflected waves, there is a limit in installing positions of the transmitter and the receiver. Therefore, there is a problem that a size of the apparatus increases.
Furthermore, the technology disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2008-164394 has a problem that a factor of a defect cannot be specified when detecting the defect in a sample. That is, it has a problem that a factor of a detected defect which may be a reduction in an elastic modulus, breakage, crack, or crease cannot be determined.